The long-term objective of this study is to understand the local population interactions in neocortex, an important process for both normal cortical functioning and for neurological disorders such as epilepsy. Our approach is to examine the spatio-temporal patterns and cellular properties of population activity in intrinsic cortical circuits. We hypothesize that one basic form of population activation in neocortex is the dynamically organized multi-neuronal ensemble. In our previous study we found a local co-activation in adult cortical slices which is related to the activity patterns in the cortex. This kind of co- activation is dynamically organized, all-or-none population event with an asynchronized, low density of spikes. It could be evoked by a proper stimulation or spontaneously occurred as a propagating activation during 7 to 10 Hz oscillations. This proposal will utilize voltage-sensitive dye imaging and electrophysiological methods to further study the spatio-temporal dynamics and neuronal activities of this cortical population event. There are four specific aims. Aim 1 will examine the proper stimulation patterns which can evoke or interact with this activity, and explore whether this activity can commonly occur in different cortical areas. Aim 2 will study the network and cellular mechanisms which sustain this co-activation. Aim 3 will study propagation of this activity and the interactions with the cortical columnar organization. Aim 4 will study the neuronal activity within this dynamically organized population event. This study will increase our knowledge about the cortical population activities related to the input pattern and intrinsic activities in cortex during normal and pathological conditions.